Below the boiling point
the substance is a liquid.
Above the boiling point the substance
is a gas.

At the boiling point, although the substance is still being heated,
the temperature does not
change for a time and the graph
becomes horizontal.
During this time the extra heat
that
is
being added
goes to overcome the forceofattraction
between the particles of the liquid as it turns into a
gas.
The process of boiling
needs
extraenergy - it is endothermic.

It is not possible to
raise the temperature of a liquidaboveits boiling
point at atmospheric
pressure.
When all of theliquid has
boiled and the liquid
has become a gas, then thetemperature of the gas
will start to rise with constantheating.